Inactivity reduces insulin action. Energy surplus causes similar reductions to insulin action. Unless energy intake is reduced to match low energy expenditure during inactivity, a concurrent energy surplus may account for the lower insulin action. This study evaluated the effect of inactivity (sitting) with and without energy surplus on insulin action. Fourteen young (26.1 ± 4.5 years (M ± SD)), lean (23.7 ± 7.1% fat), fit (VO_2peak = 49.1 ± 3.3 ml·kg^-1·min^-1) men (n=7) and women (n=7) completed each of 3, 24-hour conditions: an active condition (i.e. high energy expenditure with energy intake matched to expenditure) = ACTIV; (2) reduced energy expenditure (inactivity) with no reduction in energy intake (i.e. energy surplus) = INACTIV; (3) inactivity with energy intake reduced to match low energy expenditure = INACTIV LO-CAL. Insulin action was measured during a glucose infusion the following morning. Data were analyzed using linear mixed-effects models with planned contrasts. Compared to ACTIV, insulin action, defined as whole-body rate of glucose disappearance ( R^d) scaled to steady-state plasma insulin, was reduced 39% in INACTIV (p < 0.001) and by 18% in INACTIV LO-CAL (p = 0.07). Insulin action was also higher in INACTIV LO-CAL compared to INACTIV (p = 0.04). These results suggest that 1 day of sitting elicits large reductions in insulin action. Energy surplus accounts for half of the decline in insulin action, suggesting other factors are involved in the metabolic response to inactivity.